This invention relates to the continuous hot dip coating of wire with any of the conventional coating metals, such as zinc and its alloys, aluminum and its alloys, terne, and the like. The invention has great and particular utility in the coating of iron or steel wire with aluminum, and the embodiment of the invention specifically described will relate to this field. It should be understood, however, that no limitations are to be inferred or implied thereby.
More specifically, this invention relates to a continuous process wherein the wire passes upwardly through the bath of molten coating metal in a substantially vertical path of travel. Such a vertical mode of operation has the obvious advantage that during the period immediately after emerging from the coating bath, the force of gravity acting on the still liquid metallic coating does not tend to destroy concentricity. However, and notwithstanding this great advantage, the art has had great problems in the production of intermediate and heavy weight metallic coatings using such a vertical process.
For example, one of the earliest of the various hot dip procedures was what might be called the "free exit" method in which the wire emerged vertically from a molten metal bath which was covered with a layer of flux. In a later version of this method, the flux was replaced with a non-oxidizing gas. The thickness of the coating produced was dependent almost exclusively on wire speed, and hence it was possilbe to produce a wide range of coating thicknesses. However, the coating applied by this free exit method was very rough, and is considered unacceptable for many commercial applications. These problems are greatly exaggerated in the case of an aluminum coating metal, because of the tendency of this metal to form a tough, gummy oxide which is pulled onto the moving wire and forms a highly irregular coated surface.
U.S. Pat. Nos. 2,914,423 and 3,060,889, both in the name of Earle L. Knapp, are directed in part to a method and apparatus for improving the surface characteristics of a metallic coated wire. Both of these references include the provisions of an exit die through which the wire passes after emerging from the metallic coating bath. This exit die is completely above the normal level of the coating metal in the bath so that the moving wire will pull up the molten coating metal to the die forming an "oxide sock," while the substantially pure metal under the oxide layer is withdrawn as a coating on the wire. While this method is satisfactory for the production of light coating weights, it has been impossible to commercially produce with any degree of consistency an intermediate weight coating.
One commerical application of intermediate weight coatings is in the production of aluminum coated wire for chain link fences. Such wire, to meet the manufacturer's specification, must have a minimum of 0.40 ounces per square foot of wire surface. Because of the cost of the coating material used, it is desirable to stay as close to this minimum level as possible. In addition, the manufacturer also specifies minimum tensile strength requirements, which are equal to the breaking load of the coated wire divided by its cross sectional area. Thus, the thicker coatings make it more difficult to meet the minimum tensile strength requirements. Keeping the foregoing comments in mind, it is therefore the principal object of this invention to provide an apparatus for uniformly and consistently applying intermediate weight coatings (i.e. on the order of 0.50 ounces per square foot) to a steel wire.